03-07-2014, 02:30 PM
Sea Energy Conversion: Problems and Possibilities
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Abstract
Nowadays, policies are being developed in many
countries in order to decrease their greenhouse gases
emissions. While in this area some technologies are
widely installed (wind and solar energy), other ones, like
the sea energy, could get an important role in the medium
and long term. That is why the most relevant
technologies associated to the sea energy conversion are
presented in this paper: Tidal energy (both the traditional
power plants and those based on tidal streams), wave
energy and ocean thermal energy conversion.
However, the sea energy conversion is not completely
developed yet due to some unsolved technical problems,
apart from their high cost. The most important
advantages and disadvantages related to each kind of
technology are also analyzed in this paper, comparing the
main characteristics among them.
Therefore, technological development government
policies and the possibility of setting up a related
industrial field will be the key actions to make possible
the future of the sea energy. Solving the economical and
technical problems, it will be possible to make good use
of this alternative source of energy, with high energy
density.
Introduction
The available sea energy is enormous. If only a 10%
were used to generate electricity, it would be a significant
amount of energy regarding to the total world energy. In
fact, the possible sea energy sources and consequently
the conversion systems are divided in three main groups:
• Tidal Energy: Making use of the potential energy of
the different sea levels created by the tidal effect or
by using directly the energy of the tidal streams
• Wave Energy: It is mainly caused by the wind effect
on the sea.
• Ocean Thermal Energy: This energy is harnessed by
using a thermodynamic cycle between the different
temperatures of the ocean deep and surface water.
During the last decades, several devices and equipments
have been developed to convert the sea energy. But only
a few ones have become real-scale prototypes to be tested
on the sea. Nevertheless, an exception is La Rance
(France) tidal power plant, which has been generating
energy over the last 40 years. Apart from the tidal energy,
the other kinds of sea energies are also being tested
obtaining different results, some of them closed to the
theoretical calculations and other ones quite differents.
That is why the real condition trials are essential to
evaluate their possibilities, neither tank testing nor
computer simulation can replace them.
Besides their technical problems, economical ones should
be taken into account. Nowadays, the cost of the
produced sea energy can not compete with other
renewable energies or with the traditional ones.
Furthermore, it is usually difficult to obtain the project
funding, due to the destruction risk associated to weather
conditions.
In this paper, the most important technologies associated
to the sea energy conversion are analyzed, specifically
the following ones:
• Tidal energy (conventional power plants and
those that harness the energy of tidal streams)
• Wave Energy (shoreline and offshore
technologies)
• Ocean Thermal Energy Conversion systems
Tidal Energy
The tidal energy is the energy stored by the ocean due to
the tides produced by the combination of different
effects:
• The gravitational effect of the Sun and the Moon
• The Earth’s rotation
• Other factors, such as different ocean depths at
different places, odd shapes of the continents, Earth
tilt, etc.
Depending on the location, it is also possible to find a
once-a-day tide (one high-tide and one low-tide a day),
twice-a-day tide (two high-tides and two low-tides a
day), and so on. Therefore, tides can be considered as the
longest sea waves, with high periods (12-24 hours) and
wavelengths comparable to the length of the Earth’s
equator circumference.
Considering the different ways of using this type of
energy, the technologies are classified in the following
way:
A. Traditional Tidal Power Plants
Many centuries ago, the tidal energy was firstly used
storing water in the high tide for a later use of its
potential energy to move till wheels. Although the first
equipments on the 20th century also used this system, the
potential energy has a different application. The water
stored during the high tide by a river estuary dam is
released to move a bulb turbine, generating electricity
(i.e. La Rance Power Plant in France, figure 1). These
systems can be considered as the first generation ones.
Tidal Streams Power Plants
Today’s tendency towards tidal energy production is the
use of equipments to convert the tidal streams energy,
also called second generation systems. These systems do
not need large dams to collect the water in the reservoir.
Although in some places it is not possible to make use of
the existing tides, the tidal streams are usable to produce
energy. To this purpose, different kinds of turbines have
been developed for a submerged installation in suitable
locations.
The world available tidal stream energy is estimated at
5 TW (Isaacs and Seymur 1973), approximately the
world energy demand. Nevertheless, in spite of the
uncertainty about the existing resources, only a little part
of that energy could be used because of the streams
locations. They can usually be found near the oceans
periphery or through straits between islands or other
geological shapes. Besides, from the economical point of
view, only 1 m/s or above streams are usable for energy
generation purposes.
The most important positive factors to take into
consideration are the high predictability of the sea
streams and the high load factor of these installations
(20-60%).
The available power in tidal streams can approximately
be obtained from the following expression:
Wave energy
The ocean waves are mainly produced by the effect of the
wind (due to the sun energy) blowing over the surface of
the oceans. Considering a solar irradiance of 375 kW/m2
over the world surface, only 1 kW/m2 is transmitted to
Ocean Thermal Energy Conversion (OTEC)
The ocean thermal energy can be harnessed by means of
a thermodynamic cycle, which uses the temperature
gradient between the cold deep waters and the warm
surface waters. It is estimated that, the amount of solar
energy absorbed annually by the oceans is equivalent to
4000 times the world energy demand in the same period
[13].
The main problem with this renewable energy is the
necessity of a temperature gradient higher or equal to
20ºC, between the hot and cold reservoir. The higher the
temperature difference, the better efficiency it will have.
This requirement is only fulfilled in tropical and
equatorial zones during the whole year.
The OTEC systems efficiency is not quite high because
of the little temperature difference used in the
thermodynamic cycles. Although the ideal energy
conversion using 26 ºC and 4 ºC warm and cold
seawaters is 8%, due to several losses final 3-4%
efficiency is get [13].
Apart from thermal efficiency difficulties, long piping
systems will be necessary to pump the cold water from
1000 m depth or more.
Besides generating energy, OTEC systems can also be
used for cold water production, for mariculture, hydrogen
production and to desalinize seawater. That is why, and
due to their special conditions, in some islands this would
be a valid alternative to conventional energy power
plants. Furthermore, its only environmental impact
Comparative analysis
energy resources is showed in table 2, considering their
advantages and disadvantages in each case. The
mentioned figures must be considered as average values.
The compared characteristics are the following ones:
A. Power Unit Cost
Nowadays, the unit cost of sea energy technologies is still
higher than other kind of renewable energies. However,
R&D trends show the installation of different pilot plants,
both offshore and shoreline (also sea streams devices).
Therefore, in the medium-long term, costs are expected
to decrease and considering their good features, they
could finally be competitive with other energy resources.
Above all, OTEC systems have the highest power unit
cost (as an example, see Table 2 for a 40 MW power
plant).
Conclusions
This paper analyzes a brief description of the general
characteristics shown by the different technologies
proposed nowadays for harnessing the sea energy. In
each technology, advantages and disadvantages are
mentioned related to different factors, such as technology
efficiency, availability, environmental impact, etc.
Finally, a comparative analysis is done to get an idea
about the state-of-the-art of the sea energy conversion
systems.